Process for Electroless Deposition of Metals Using Highly Alkaline Plating Bath

ABSTRACT

A plating process using an electroless plating bath formed from two separate prepared component solutions. The component solutions mixed within 120 hours prior to plating operations, to provide a highly alkaline plating bath solution. One component solution of the two-part plating bath, is provided with a metal salt or source of plating ions, and which is initially kept in a separate solution from the second other prepared component solution. The second component solution contains formaldehyde, and preferably paraformaldehyde, used to reduce the metal salts into the metal to be deposited on a substrate. Each component solution further includes sodium hydroxide in concentrations selected so that when the two solutions are preferably mixed the final plating bath solution has a pH greater than 11.5.

RELATED APPLICATIONS

This application claims the benefit of 35 USC 119(e) to U.S. Provisional Patent Application Ser. No. 61/344,800, filed 13 Oct. 2010; U.S. Provisional Patent Application Ser. No. 61/457,446, filed 30 Mar. 2011; and U.S. Provisional Patent Application Ser. No. 61/457,590, filed 26 Apr. 2011.

SCOPE OF THE INVENTION

The present invention relates to a process for the electroless deposition or plating of metals on substrates, and more particularly a process for coating metals and metal alloys soluble at high pH levels greater than 11.5, and most preferably which are soluble at a pH of between 13.5 and 14.

BACKGROUND OF THE INVENTION

In electroless coating processes, it is generally known that plating baths with alkalinity of higher values of pH of 11 or more are detrimental, since the rate of deposition falls off dramatically and the usable life of the electroless plating bath solution is shortened. As a result of the shortened solution life, heretofore the use of highly alkaline electroless plating solutions have not proven commercially viable. Furthermore, experiments with highly alkaline plating solutions usually show that metal deposition may stop at pH levels of 13 or more.

The applicant has appreciated that in the case of more reactive substrates, magnesium is a very light metal readily available material with good structural and mechanical properties, making it an ideal replacement for other heavier metals. A major issue with magnesium plating is that when it is mechanically attached to other metals an electrical conductivity exists between the two metals and the resulting galvanic effect may result in rapid oxidation corrosion of the magnesium.

With more highly reactive substrate materials such as magnesium, conventional electroless deposition solutions are susceptible to produce coatings which are intermittent due to surface oxidization and/or corrosion of the substrate surface disrupting the electroless deposition process. As such, conventional electroless plating solution baths suffer disadvantages in that they permit and even facilitate the oxidation and/or corrosion of the reactive metal substrate surfaces, resulting in at most, spotty depositions of the desired coating.

To date the best solution to prevent the oxidization corrosion of magnesium has been to electrically isolate the magnesium from any other metal contact. Such isolation plating systems are restricted to limited applications, which in turn limits the use of magnesium.

SUMMARY OF THE INVENTION

To overcome at least some of the disadvantages associated with conventional electroless plating processes, the present invention contemplates a plating process which uses an electroless plating bath formed from two separate prepared component solutions. The component solutions are preferably separately formed, and thereafter mixed shortly before and preferably within about five days prior to plating operations, to provide a highly alkaline plating bath solution which has a pH greater than about 11.5, preferably greater than about 13, and most preferably between about 13.5 to 14.

One component solution of the two-part plating bath is provided with a metal salt or source of plating ions, and which is initially kept in a separate solution from the second other prepared component solution containing a formaldehyde, and paraformaldehyde, used to reduce the metal salts into the metal to be deposited on a substrate. Each component solution further includes sodium hydroxide in concentrations selected so that when the two solutions are mixed in a ratio of about 0.5:1 to 1.5:1, and preferably 0.7:1 to 1:1 the mixture provides an alkaline final plating bath solution having a pH greater than 11.5, preferably greater than 13, and most preferably of between about 13.5 to 14.

Preferably, the individual component solutions are prepared as pre-prepared solutions which are physically separated from each other until upto seven days, and preferably upto at least 3.5 days prior to plating operations.

The applicant has appreciated by a process in which the plating bath solution is prepared by mixing two pre-prepared component solutions, it is possible to extend the shelf life of the individual bath components, increasing their stability, enabling their use in larger scales commercial electroless plating processes. In particular, the applicant has appreciated that with conventional highly alkaline plating solutions having pH levels in excess of 11.5, the sodium hydroxide in the plating bath may result in precipitation of the plating metal, shortening the bath shelf life. By maintaining plating bath component solutions separate, both component solutions may be pre-prepared for later mixture to provide a high pH plating bath in either a batch or as part of a continuous commercial plating process.

The applicant has appreciated that with the present process, in one embodiment magnesium may advantageously be plated with a selected plating metal in a highly alkaline plating bath having a pH of about 13.5 to 14, thereby avoiding any galvanic effect on the magnesium when mechanically attached to other metals. Thus the present process allows magnesium to be isolated from galvanic coupling while, at the same time, maintaining electrical conductivity through the magnesium structure. The present electroless deposition process contemplates that magnesium could therefore be used in a variety of structures and assemblies where dissimilar materials are mechanically fastened together.

The applicant has appreciated that electroless cladding of plating metals, such as copper in a highly alkaline deposition bath established for magnesium, magnesium alloys and other reactive metals is not limited. In an alternate embodiment, the present process is also suitable for use in plating metals such as copper on silicon substrates. Unlike with magnesium, the electroless deposition of metals ON silicon does not require a highly alkaline bath to mitigate corrosion. Rather, the higher alkalinity achieves improved electroless copper deposition at pH levels in excess of 11.5, and more preferably at a pH level of around 13.5, in an environment containing an excess of formaldehyde derivative reducing agent, such as paraformaldehyde.

The high pH electroless coating bath of the present invention advantageously can also be used to apply coating of other metals and alloys to a variety of different substrates. Further substrates include, but are not limited to: beryllium, vanadium, and titanium. The high pH electroless coating process is useful with a range of plating or coating metals and alloys including but not limited to: silver, copper, nickel/tungsten, nickel, boron and any other metals and alloys that are soluble at higher pH levels greater than 11.5, preferably greater than 13, and most preferably at between 13.5 to 14.

In electroless coating systems using the two-part plating bath of the present invention can also include doped hybrid solutions containing non-metallic particles such as but not limited to, diamond, Teflon™, ceramics, and/or molybdenum.

Accordingly, in one aspect, the present invention reside in a process for electroless plating a plating metal on a substrate comprising: preparing a first bath solution comprising: 10 to 50 g/L sodium hydroxide, 40 to 120 g/l potassium sodium tartrate, and a metal salt, preparing a second bath solution physically separates from the first bath solution comprising: 40 to 75 g/L paraformaldehyde, and 30 to 50 g/L sodium hydroxide, mixing the first and second bath solutions to form a mixed plating bath solution having a pH greater than 11.5, and immersing a substrate to be plated in the mixed solution, wherein the metal salt comprises a plating metal selected from the group consisting of Cu, Al, Ni, Au, Ag and their alloys.

In another aspect, the present invention resides in a process for electroless plating nickel or a nickel alloy plating metal on a magnesium metal substrate comprising: preparing a first bath solution comprising: 25 to 60 g/L nickel chloride hexahydrate, preparing a second bath solution physically separate from the first solution comprising: 40 to 75 ml/L ethylenediamine, 30 to 50 g/L sodium hydroxide, and 3 to 8 g/L sodium borohydride, mixing the first and second bath solutions to form a mixed plating bath solution having a pH of at least 12, and immersing a substrate in the mixed solution

In still a further aspect, the present invention resides in a process for electroless plating a plating copper on a substrate comprising: preparing a first bath solution comprising: 15 to 25 g/L sodium hydroxide, 60 to 100 g/L potassium sodium tartrate, and 35 to 40 g/L CuSO₄.5H₂O, preparing a second bath solution component physically separated from the first bath solution component comprising: 50 to 65 g/L paraformaldehyde, and 20 to 45 g/L sodium hydroxide, mixing the first and second bath solutions in a ratio selected at between about 0.5:1 to 1.5:1 to form a mixed plating bath solution having a pH greater than about 13, and with said bath having an operating temperature between about 17° C. and 32° C., immersing a substrate to be plated in the mixed solution.

In a further aspect, the present invention resides in a process for electroless copper plating on a substrate comprising: preparing a first bath component solution comprising: 10 to 30 g/L sodium hydroxide, 40 to 120 g/l potassium sodium tartrate, and 20 to 45 g/L copper sulfate pentahydrate, preparing a second bath component solution physically separates from the first bath solution comprising: 40 to 75 g/L paraformaldehyde, and 20 to 50 g/L sodium hydroxide, mixing the first and second bath solutions to form a mixed plating bath solution having a pH greater than 13, and immersing a substrate to be plated in the mixed solution, wherein the substrate comprising, a metal selected from the group consisting of magnesium, aluminum and their alloys.

In yet a further aspect, the present invention resides in a process for electroless plating Nickel-boron plating metal on a magnesium substrate comprising: preparing a first bath solution comprising: 25 to 50 g/L nickel chloride hexahydrate, preparing a second bath solution component physically separate from the first bath solution component comprising: 50 to 75 ml/L ethylenediamine, 30 to 50 g/L sodium hydroxide, and 3 to 8 g/L sodium borohydride, mixing the first and second bath solution components in a ratio selected to form a mixed plating bath solution having a pH of at least 13, and preferably about 14 immersing the magnesium substrate in the mixed solution.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A highly alkaline plating bath solution for use in metal deposition is prepared in two component parts: the first component part being the metal-salt solution (solution A); and the second component part being the solvent part (solution B) of the final bath solution. Each of the component solutions include sodium hydroxide in concentrations selected to maintain the stability of each component while allowing their mixture to provide a final highly alkaline plating bath suitable for substrate use in plating both reactive metals, as well as silicon-based. The two component solutions are prepared and stored separately until shortly before they are to be used in the process. In particular, preferably within 84, and more preferably within 72 hours prior to plating operations, the component solutions are mixed at the desired ratios to form the final electroless metal deposition bath for processing parts.

The use of separately prepared and stored component solutions allows the finalization of highly alkaline electroless copper deposition bath having a pH greater than about 13, and preferably from about 13.5 to about 14, and which in the final solution contains very low levels of hydrogen. More preferably, in the solvent component solution, in place of formaldehyde which is traditionally used as the active chemistry of the anodic reaction, paraformaldehyde (the smallest polyoxymethylene) is provided in the solvent solution.

The present process is believed to achieve various advantages, particularly in, although not limited to, the plating of magnesium and magnesium alloys. With the present process, the total encapsulation of a magnesium substrate in another metal that is not subject to galvanic oxidation will prevent galvanic oxidation of the magnesium core of the part.

In addition, the two-part component solutions that combine to form plating the bath for high pH electroless plating processes, maintain individual stability and permit long-term storage.

In addition high pH deposition bath prevents highly reactive metal substrates to be plated from oxidizing prior to the desired metal deposition being placed on the surface. As such, the high pH of the plating bath provides an environment where complete surface coverage of a highly reactive material may take place prior to any significant oxidation of surfaces which could otherwise prevent the formation of the desired coating.

Using the two-part component solutions, experimental highly alkaline electroless plating baths were prepared as follows:

Example 1 Copper Cladding of Magnesium/Magnesium Alloys

In the case of direct electroless copper cladding of magnesium alloys from a highly alkaline deposition bath, a deposition bath was prepared by mixing a component metal-salt solution and solvent solutions prepared generally as follows:

TABLE 1 Bath Composition Formula Amount Bath A Copper CuSO₄•5H₂O 35.0 g/L-45.0 g/L Sulfate Pentahydrate Potassium KNaC₄H₄O₆•4H₂O 60.0 g/L-11.0 g/L sodium tartrate (Rochelle's Salt) Sodium NaOH 10.0 g/L-30.0 g/L Hydroxide Bath B Parafor- HO(CH₂O)_(n)H_((n =) 50.0 g/L-70.0 g/L maldehyde ₈₋₁₀₀₎ Sodium NaOH 20.0 g/L-45.0 g/L Hydroxide Operating Temperature: pH = 14 20-25° C. Note: A wider range of components within the deposition bath are possible as, unlike magnesium, the substrate does not corrode within the deposition bath.

Sample Preparation:

The substrates used were AZ91D and AM50 magnesium alloys (their composition is given in Table 2) that were cut into coupons of 2 cm×3 cm×0.5 cm. The samples had a hole drilled at the top on the 2×3 cm face so that they could be hung in the deposition bath via non-conducting nylon wire. To ensure a uniform initial surface, samples were wet-polished smooth using 240 grit SiC emery paper and rinsed in distilled water.

Sample Deposition Bath:

A highly alkaline deposition baths used for sample electroless deposition of Cu was prepared according to Table 3.

TABLE 2 Table 2: The compositions of AZ91D and AM50 magnesium alloys (in wt. %) Alloy Al Zn Mn Ni Cu Si Fe Magnesium AZ91D 8.3-9.7 0.35-1.0 0.15 <0.002 <0.03 <0.10 <0.005 Balance AM50 4.9 0.2 0.45 <0.01 <0.008 <0.05 <0.004 Balance

Solution A

In Solution A for every litre of de-ionized water the following was added:

TABLE 3 Composition Formula Amount pH Copper Sulfate CuSO₂•5H₂O  40.0 g/L pH ≈ 14 Pentahydrate Potassium Sodium Tartrate KNaC₄H₄O₆•4H₂O 100.0 g/L Sodium Hydroxide NaOH  25.0 g/L

Solution B

In Solution B, for every litre of de-ionized water the following was added:

Composition Formula Amount pH Paraformaldehyde HO(CH₂O)_(n) H_((n = 8-100)) 65.0 g/L pH ≈ 14 Sodium Hydroxide NaOH 40.0 g/L

Each of component solutions A and B exhibited high stability and an extended shelf life at room temperature. Following their preparations, component solutions A and B were mixed, the resulting mixed electroless plating solution was found to have both a high pH of between about 13.5 and 14; and a useful working life of at least 48 hours at room temperature. The shelf life of the mixed plating solution to be used was however found to depend on the temperature of the solution; the amount of processing through the solution; and the ratio of solution A to B. These variables have the following effect on the life of the solution:

1. The higher the temperature the shorter the active solution life.

2. The higher the processing load, the shorter the active life of the solution.

3. The higher the ratio of solution B to A, the shorter the active solution life.

Deposition Procedure:

Samples were dry polished in open atmosphere using 240 grit SiC emery paper to remove the quick forming oxide/hydroxide layer. The polishing was done such that minimal heating of the sample occurred as to not further promote the formation of the insulating oxide layer. The samples were then placed into the electroless deposition bath (made of a 1:1 mixture of baths A and B) at temperatures prescribed in Table 1.

After sufficient deposition time, the samples were removed and rinsed with distilled water before being hung dry. To increase the rate of drying, it was found beneficial to have the base of the samples in contact with a non-conducting wire as to allow for the flow of adsorbed water away from the sample.

Results:

The first sets of deposits performed on the AZ91D Mg alloy were conducted to isolate the role of the oxides. Two samples were placed parallel in identical room temperature electroless Cu plating baths. First, however, both samples were wet polished, using 240 grit SiC paper, and let dry open to the atmosphere for 3 weeks. One of the samples then underwent polishing and was placed into the deposition bath as quickly as possible, as per our procedure, while the other was left untreated. Finally, both samples were left in the deposition bath for 20 minutes before removal. Deposition on the oxidized sample was essentially non-existent, while the deposit on the coated sample was of better quality being more continuous both macroscopically and as seen by EDS.

Using the aforementioned two-part solution, a commercial process for plating magnesium/magnesium alloys is provided where the component solutions A and B are mixed as either a batch process or as part of a continuous electroless plating system as follows:

-   -   1. The surface of the magnesium alloy to be coated is prepared         by removing the oxide surface coating. Surface oxides may be         removed mechanically through various abrasion processes; through         a chemical dip process; or by plasma.     -   2. The surface of the alloy substrate to be plated is then         cleaned.     -   3. Once the oxide coating has been removed, the exposed         magnesium alloy will start to oxidize upon exposure to air. It         is therefore highly preferred that the alloy substrate with a         prepared surface be immersed in the coating bath in less than 30         minutes of oxide removal.     -   4. The magnesium surfaces to be coated are preferably fully         submerged into the deposition bath for 15 to 30 minutes, based         on the thickness of copper coating desired.         -   The rate of copper deposition on magnesium is depended on             the cumulative effect of the following factors and             variables:             -   a. Rates of coating built increased as the temperature                 of the bath was increased. At this point the full range                 of available temperatures have not yet been accurately                 tested, but due to the nature of the material there is                 an upper limit that will be defined with additional                 experimentation. Further, there will be an ideal                 temperature window that will produce the greatest amount                 of coating deposition per bath in the least time.             -   b. Surface area being coated will affect the rate of                 deposition. The greater the surface area in a bath of                 given volume reduces the overall rate of deposition.             -   c. Amount of copper left in solution that is available                 for subsequent coating. This level of copper is reduced                 by the amount of copper coated onto part surfaces by                 this solution and by the age of the solution where over                 time the solution looses copper content through copper                 precipitating out of solution naturally through various                 factors.     -   5. After immersion in the plating bath for a desired period, the         plated substrate is removed from bath and the copper coated         magnesium alloy part is rinsed in water or sodium hydroxide         solution. In general, the water rinse will result in a bright         copper finish. A hydroxide rinse solution may be used to provide         the copper plated article a darker appearance to the finish.

Example 2 Pre-Treatment—Acidic Removal of Oxide Layers on (AM50 & AZ91D) Mg Alloys

In more preferred plating metal, acid etching is preformed on the substrate as a pretreatment to provide enhanced electroless copper deposition and bonding. It has been previously documented that acidic etching is capable of removing insulating oxides for the surface of a variety of metals including aluminum [Al] and magnesium [Mg]. Additionally, it has also been documented that the use of some acids are not conducive for oxide removal in the face of secondary deposition, as they result in corrosion of the substrate.

In the case of Mg alloys it is know that corrosion occurs in the presence of chloride [Cl⁻] and sulfate [SO₄ ²⁻] anions, which in turn may provide for preferential corrosion zones to form. In the case of electroless copper [Cu] deposition, the quazi-crystalline structure of the amorphous Cu deposits results in a difficulty in plating regions where corrosion from anionic components have begun.

To provide enhanced plating properties, tartaric acid [C₄H₆O₆] Table 5 and sulfuric acid [H₂SO₄] (Table 6) were tested for the removal of oxides from Mg alloy surfaces. Test samples were dry polished with 240 grit SiC emery cloth and allowed to oxidize in open air over 48 hours prior to exposure to the de-oxidation treatments. In both cases, it was found that the acid was able to remove the oxide layer and allow for better deposition. In test examples, acidic exposure was limited to only a few seconds, and no rinse bath was conducted between the oxide removal and deposition steps, as the distilled water bath would result in re-oxidation of the surface.

Additionally, the addition of cupric sulfate pentahydrate [CuSO₄.5H₂O] was attempted in the C₄H₆O₆ bath in accordance with Table 7. In this case it was found that a simple displacement reaction appeared to occur with a black, discontinuous copper film appearing to form on the Mg-based substrate. Though the black deposit from the treatment was not very well adhered, subsequent copper deposition appeared to be very well adhered, though at a cost of bath life.

The baths used in pre-treatment with acidic removal of oxide layer and subsequent electroless copper plating were prepared as follows according to Table 4:

TABLE 4 Tartaric Acid + Copper Sulfate Pentahydrate Bath Chemical Formula Concentration Tartaric Acid C₄H₆O₆ 53.0 g/L Copper Sulfate Pentahydrate CuSO₄•5H₂O 30.0 g/L Note: Tartaric acid has some solubility issues at the concentration of 53 g/L precipitating a white substance at the bottom of the vessel when paired with 30 g/L CuSO₄•5H₂O.

TABLE 5 Tartaric Acid Bath Chemical Formula Concentration Tartaric Acid C₄H₆O₆ 53.0 g/L

TABLE 6 Sulfuric Acid Bath Chemical Formula Concentration Sulfuric Acid H₂SO₄ 20 mL/L

TABLE 7 Electroless Copper Bath Bath Chemical Formula Concentration Bath A Sodium Hydroxide NaOH 25.0 g/L Potassium Sodium KNaC₄H₄O₆•4H₂O 60.0 g/L Tartrate (Rochelle's Salt) Copper Sulfate CuSO₄•5H₂O 35.0 g/L Pentahydrate Bath B Sodium Hydroxide NaOH 25.0 g/L Paraformaldehyde HO(CH₂O)_(n)H_((n = 8-100)) 65.0 g/L

The results suggest strongly that most acids are not sufficient to activate oxidized silicon surfaces, as nitric [HNO₃] and sulfuric [H₂SO₄] acids at a concentration of 20 mL/L were ineffective to alter the surface in any way, even after 5 minutes.

Once magnesium substrates have been encapsulated in a metal alloy coating, the metal coating can thereafter itself serve as the basis for the application and deposition of subsequent coatings. Selection of the initial coating is predicated on the subsequent coatings desired. Further, metal encapsulated magnesium maintains its electrical conductivity and may be fastened mechanically to dissimilar metals without galvanic effect or corrosion at the point of fastening.

Example 3 Nickel-Boron (Ni—B) Cladding of Magnesium/Magnesium Oxide

An electroless deposition coating solution nickel-boron metal coating on magnesium substrates was prepared from a two-part solution (component solution A and component solution B) shown in Table 8 that was mixed just prior to substrate coating. As nickel itself is not soluble at high pH, a nickel-boron salt solution was prepared in a separate batch solution A maintained at a much lower pH than the final bath which forms part A which is shown in the table below.

The nickel-boron deposition solution is prepared as a two-part system mixed with de-ionized water as follows:

TABLE 8 Bath Composition Formula Amount pH Bath A Nickel NiCl₂•6H₂O 30.0 g/L pH ≈ 5 Chloride Hexahydrate Bath B Sodium NaOH 40.0 g/L pH ≈ 14 Hydroxide Ethylene- H₂NCH₂CH₂NH₂ 60.0 mL/L diamine Sodium NaBH₄  4.8 g/L Borohydride Bath Operating Temperature for Deposition final pH ≈ 14 80-95° C.

The solvent component solution B included the borohydride which is highly susceptible to oxidation in neutral pH or acidic pH solutions. As a result when the solutions A and B are mixed for final use, solution A is added to solution B to advantageously prevent the oxidation of the borohydride. The ethylenediamine in compound solution B further facilitates the solubility of the nickel in the high pH solution and the nickel deposition on the magnesium surface; while the boron is deposited on the surface by means of the anodic reaction.

It should also be noted that ethylenediamine is highly reactive with copper. As a result copper is preferably avoided during nickel-boron coating. The nickel-boron coating may however, be subsequently coated with copper by way of the electroless coating process described herein.

The electroless deposition of nickel-boron as a plating on a magnesium substrate is performed as follows:

-   -   1. Each of component solutions A and B are prepared as         physically separate solutions.     -   2. Following preparation, the solutions A and B are mixed at         room temperature, by pouring solution A into solution B, then         heated as a single plating bath to a temperature between 80 and         95° C.     -   3. The magnesium substrate to be coated is cleaned either         mechanically, chemically, or plasma to remove the oxide surface         from that magnesium.     -   4. Subsequent to the removal of the oxide, oxide-free surface         parts of the magnesium substrate are subjected to secondary         cleaning by washing with de-ionized water.     -   5. The cleaned magnesium substrate is then submerged in the         prepared plating bath to allow the solution to have access to         all surface area of the part for up to 30 minutes (but not         limited thereto), depending on the final thickness of the         coating to be achieved.     -   6. After the desired thickness of coating has been deposited on         the magnesium part the part is rinsed in water or as sodium         hydroxide solution.     -   7. The addition of cobalt and/or zinc ions into the bath may         furthermore achieve additional beneficial effects     -   In the nickel-boron of magnesium, the rate of deposition is         depended on the cumulative effect of the following factors and         variables:         -   a. Ratio of component solution A to solution B; with             solution A controlling the volume of metal salt present; and         -   b. The temperature of the bath (i.e. between 80 and 95° C.).

Sample Preparations:

A trial using an 85-90° C. Ni—B deposition bath for 5 minutes also was able to produce a deposit on a sample of AZ91D magnesium alloy. In the case of Ni—B the temperature is one important factor in the formation of the coating, with a lower temperature significantly slowing the deposition rate. Though the macroscopically seen continuous coverage was observed to be microscopically discontinuous using scanning electron microscope (SEM) and energy dispersive x-ray spectroscopy (EDS), some of the perceived discontinuity from EDS is believed due to a lack of film thickness. Nevertheless, deposits of this degree of continuity are sufficient to provide the minimum basis for secondary subsequent deposits at high pH; the high pH being necessary to mitigate any exposed magnesium which would react harshly in an acidic electrolyte.

Deposition of a secondary electroless Cu thin film coating on Ni—B was observed to provide a nearly continuous coating both macroscopically as well as seen by SEM. The initial Ni—B deposit was produced on AZ91D Mg alloy over 5 minutes at around 87° C., resulting in a deposit with some expected discontinuity. After a distilled water rinse and 15 minutes of drying, hanging in open air, the sample was placed in a room temperature electroless copper bath for 5 minutes after which the sample was rinsed in distilled water and again hung to dry open to atmosphere. Observations of the sample after the secondary deposition process suggest that the Ni—B coating was indeed somewhat discontinuous as some limited corrosion has occurred on the sample. This is confirmed by SEM which indicates an initially discontinuous coating that has only begun to build on the Ni—B ‘nucleation’ sites.

A further sample attempt of the same using a 15 minute Ni—B deposit at around 80° C., distilled water rinse, 7 minutes of drying, and a room temperature electroless copper bath for 22 minutes resulted in a better coating with only few defects observed by SEM and EDS. Observations of the sample during the secondary deposition process suggest that the Ni—B coating was discontinuous, as the Cu deposit did not appear as bright and after 7 minutes deposition time in electroless copper produced a relatively thin deposit. The second sample showed a definite increase in the continuity with the SEM image of the surface clearly exhibiting coverage over the wear tracks from the polishing process.

To compare the initial layer with the morphology of a second layer, a sample was produced such that only the lower half was exposed to secondary Cu deposition bath. The initial electroless Ni—B deposit was produced on a polished sample of AZ91D alloy at 89° C. over 5 minutes. As seen by SEM, the deposit was continuous with few defects only, even though EDS continued to show a small peak of Mg. The sample was then rinsed in distilled water and hung to dry for 25 minutes in open air. After the drying period, the lower part of the sample was exposed to a room temperature electroless Cu bath for another 5 minutes then rinsed and dried. During the secondary deposition process the coating hydrated above the bottom third exposed to the deposition bath. As this deposit was achieved in about a quarter of the time needed for the initial sample, it demonstrates that the initial coating need not be entirely continuous for good secondary deposits to be produced; though continuity of the primary layer is a factor in the quality of the secondary deposit. Additionally, SEM analysis of the secondary Cu cladding demonstrated an improved sequestration of magnesium. Macroscopically some corrosion may be observed about the Ni—B/Cu interface and is rationalized as galvanic corrosion due to the incomplete immersion of the sample.

The electroless deposition process in highly alkaline environments provides well formed, well adhered deposits, especially in the case of the deposition of a secondary layer. Presently, secondary deposition baths are also highly alkaline in order to ensure any pinholes, gaps, or defects in the otherwise continuous coating do not readily form galvanic cells and begin corrosion. Gaps in the initial cladding of magnesium may be attributed, especially in the case of copper, to the formation of insulating surface oxides by means of one, or both, of the following likely processes.

1) The alloys tested thus far have been AZ91D and AM50 magnesium alloys which nominally contain aluminum at around 9% and 5%, respectively. As aluminum readily oxidizes within a highly alkaline environment, it is expected that some oxidation of Al—Mg intermetallics may occur within the deposition bath leading to the formation of the insulating oxides. In this case, it is expected that non-aluminum alloyed magnesium alloys would perform better in the highly alkaline deposition baths. This would also be indicative of a significant development in the cladding of magnesium alloys as, to date, no single cladding procedure is sufficient for coating a large variety of magnesium alloys.

2) Polishing the magnesium alloys in open air results in the potential heating of the surface, particularly the asperities, which contribute to the promotion of oxidation. Tests on insulating role of oxides have confirmed that these electroless deposition techniques are ineffective in forming electroless claddings on oxidized magnesium. Though this issue could easily be resolved by either cooling the sample, or polishing under inert gas atmosphere to prevent oxidation, our method has shown that with proper care the formation of insulating oxides can be managed.

The ability to deposit copper, despite the stark difference in standard electrode potentials between copper (+0.340 vs. SHE) and magnesium (−2.372 vs. SHE), is a consequence of the highly alkaline deposition bath. The highly alkaline deposition bath helps combat corrosion of the substrate and the formation of aggressive galvanic cells. This has been observed with deposits produced at the slightly more acidic pH values (pH≈12) resulting in corrosion of the substrate. For this reason, the solubility of copper in highly alkaline environments was a significant factor in the selection of copper as cladding metal.

Another important observation is that exposure of a polished Mg alloy sample to Bath A of the copper deposition bath results in the formation of well adhered copper on the surface even without the presence of reducing agent.

Example 4 Electroless Deposition of Copper on Aluminum Alloy Substrate

In another embodiment, the process of the present invention may be used in the electroless deposition of metal coating layers such as copper or an aluminum or aluminum alloy substrate.

The copper deposition plating bath is prepared on a two-part bath from component solutions A and B as follows:

Copper Deposition Bath

TABLE 9 Bath Composition Formula Amount Bath A Copper Sulfate CuSO₄•5H₂O 40.0 g/L Pentahydrate Potassium KNaC₄H₄O₆•4H₂O 70.0 g/L-100.0 g/L Sodium Tartrate (Rochelle's Salt) Sodium NaOH 20.0 g/L-25.0 g/L Hydroxide Bath B Paraform- HO(CH₂O)_(n)H(_(n =) 65.0 g/L aldehyde ₈₋₁₀₀₎ Sodium NaOH 40.0 g/L Hydroxide Operating Temperature: pH = 14 20-25° C. In the coating of copper on an aluminum alloy substrate, the following procedure is provided.

-   -   1) Any oxide layer is removed from the Al alloy surface by         conventional means. Most preferably oxide removal is performed         by methods which also increase the surface roughness of the         substrate, such as dry polishing, to improve adhesion.     -   2) The Al alloy is placed into the room temperature electroless         copper deposition bath for about 5-10 minutes. Longer deposition         times and higher temperatures may be utilized to increase the         deposition rate and/or coating thickness.     -   3) Following formation of the copper layer, the plated sample is         removed from the deposition bath and rinsed in distilled water         to remove excess electrolyte.     -   4) The result is a bright, even/uniform, continuous, well         adhered electroless copper cladding of the Al alloy.

The sample upon which the electroless Cu deposition took place in accordance with the most preferred method required polishing, with little deposition occurring on the unpolished surface.

Other Al alloy samples have seen deposition on both oxidized and polished surfaces with the deposits being poorly adhered and/or powder-like. It is envisioned that the deposit characteristics may be used as a test to determine whether a well adhered deposit may take place with polishing.

The electroless deposition on Al alloys from a high pH deposition bath is believed to be counterintuitive, as aluminum is generally understood to oxidize rapidly (spontaneously) in a hydroxide environment, as shown by a positive standard electrode potential, E⁰.

Al_((s))+3OH⁻ _((aq))→Al(OH)_(3(s))+3e ⁻ E ⁰=+2.31V

With the present invention, electroless deposition of copper is reported to be at a maximum pH of 13.5 for electroless copper baths, with a high concentration of formaldehyde reducing agent on an AlN substrate.

Other electroless copper procedures which may provide suitable for Al plating include a copper immersion coating on 3003-Al alloy, prior to electroless Ni—P deposition. The copper immersion coating is formed in a bath with CuSO₄ 5H₂O (30 g/l), and C₄H₆O₆ (tartaric acid) (53 g/l) at 25° C. for 3 min. This coating is done to prevent direct contact of Al with a subsequent electroless nickel deposition solution, thereby increasing the stability of the electroless deposition bath.

The applicant has appreciated that using the present invention, the immersion coating may be a way to expand the electroless copper deposition to a wider range of aluminum alloys. In this regard, a subsequent electroless copper layer could be acidic or alkaline, as subsequent electroless Ni—P coating layers may be provided at conventional pH levels of about 4.5.

Example experiments achieve and show an adhered electroless copper cladding on an Al alloy sample. In the test sample the Al was a highly recycled metal with any number of impurities entering the mix. Test conducted on 12% Si and 6061 Al alloys have resulted in powdered, poorly adhering deposits on the surface of the sample. As such the nature of the alloy itself may be a contributing factor which allows a high degree of deposition to take place.

On other samples tested, deposition took place on the oxidized surface as well as the polished region, with deposition lacking independent of polishing.

While the preferred method describes emersion in an electroless copper deposition bath having a pH in excess of 13, and preferably 13.5 to 14, it is contemplated that the electroless deposition of copper may be achieved at lower pH values, depending on the specific composition of the aluminum alloy and whether adhesion may be lacking. Lower pH electroless Cu would also be effective, possibly on those alloys upon which adhesion is lacking.

As one possible commercial application, within solar cells, the electroless copper process may be applied to form the conductive backing used to reunite ‘electrons’ with the ‘holes’. Conventionally conductive backing is currently made from an aluminum paste. More importantly, the electroless deposition process of the present invention may be used to apply a copper layer to form the electrode gridding contacts on the front surface of the cell. Currently, the electrodes are formed by screen printing silver paste on both the front and back of the solar cell. The use of an electroless copper plating process may be both less expensive than using silver paste, instead of conventional printing processes could further increase solar cell efficiency by reducing the surface area currently covered by the gridding.

Example 5 Electroless Copper Deposition Silicon Substrate

The present electroless technique also shows promise in to integrated circuit manufacture, and in particular the assembly of processors.

In experimental results, the deposition of copper using the process of the present invention has also been verified on n-type silicon substrates, as well as on another silicon sample believed to be essentially pure silicon. Given that the silicon is doped to form n- and p-types of silicon it is expected that the deposition technique will work on all silicon substrates used in the construction of electronic devices. Additionally, it should be noted that copper deposition was observed on the substrate edge where the sample was broken off from a bulk sheet indicating that it is the lack of oxide, and not some anomaly from the polishing method, that results in deposit formation.

Measurement of the coating deposit thickness, and whether the deposition bath promoted oxide growth, forming an oxide interlayer between the silicon substrate and the copper cladding, will further allow for the adjustment of optimum bath conditions. Preliminary measurements appear to indicate that there is some degree of ohmic contact between the cladding and the substrate measurement using the 4-probe method of thin films is however required to verify accuracy.

The electroless deposition of coating layers will allow a variety of silicon or metal substrates to be used in a great number of areas and applications which are not considered to be viable to date. By way of non-limiting examples, these include but are not limited to the use of coated magnesium/magnesium alloy substrates, computer hard drives, naval vessels, aircraft and aerospace applications, internal combustion engine heads and blocks, transmission and gear housings, automobile frame assemblies, and the like.

Further, the amount of metal deposited on the surface of a given substrate should not affect its recycling. In particular, surface coatings may be applied in such controlled volume to remain within the limits of acceptable “impurities”. Further, high wear resistant or hardened coatings may be applied on softer metal substrates such as magnesium, which will allow the metals to be used in areas where good surface wear qualities are required.

Although the detailed description describes the process of the present invention, as used in copper and Nickel-Boron coating of magnesium, aluminium and silicon substrates, the invention is not limited. It is to be appreciated that the two-part coating process of the present invention may be used to apply a variety of different coating layers which are soluble in highly alkaline plating baths.

While the detailed description discloses various preferred plating method parameters, the invention is not so limited. Many variations will now be apparent. For a definition of the invention, reference may be had to the appended claims. 

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 22. A process for electroless plating nickel or a nickel alloy plating metal on a magnesium metal substrate comprising: preparing a first bath solution comprising: 25 to 60 g/L nickel chloride hexahydrate, preparing a second bath solution physically separated from the first bath solution comprising: 40 to 75 ml/L ethylenediamine, 30 to 50 g/L sodium hydroxide, and 3 to 8 g/L sodium borohydride, mixing the first and second bath solutions to form a mixed plating bath solution having a pH of at least 12, and immersing a substrate in the mixed solution.
 23. The process of claim 22, wherein the plating metal is a Nickel-Boron alloy.
 24. The process of claim 22, wherein the mixed solution has a pH of at least 13 and the metal substrate is immersed in the mixed solution for a period of between about 1 and 60 minutes.
 25. The process of claim 22, wherein the mixed solution is maintained at a temperature selected at between about 80 and 95° C. during immersion of the substrate therein.
 26. The process of claim 22, wherein the first and second bath solutions are mixed in a continuous batch process, wherein the first and second bath solutions are mixed in an approximately 1:1 volumetric ratio.
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 37. A process for electroless plating Nickel-boron plating metal on a magnesium substrate comprising: preparing a first bath solution comprising: 25 to 50 g/L nickel chloride hexahydrate, preparing a second bath solution component physically separate from the first bath solution component comprising: 50 to 75 ml/L ethylenediamine, 30 to 50 g/L sodium hydroxide, and 3 to 8 g/L sodium borohydride, mixing the first and second bath solution components in a ratio selected to form a mixed plating bath solution having a pH of at least 13, immersing the magnesium substrate in the mixed solution.
 38. The process of claim 37, wherein during immersion of the magnesium substrate, the mixed plating bath is maintained at an operating temperature selected at between about 80° C. and 95° C.
 39. The process of claim 37, wherein the magnesium substrate is immersed in the mixed solution for a period of between about 1 and 60 minutes.
 40. The process of claim 37, wherein the first bath solution component is physically separated from the second bath solution component for a period of at least 5 hours.
 41. The process as claimed in claim 40, wherein the first bath solution component is physically separated from the second bath solution component for a period of at least 72 hours.
 42. The process of claim 23, wherein the mixed solution has a pH of at least 13 and the metal substrate is immersed in the mixed solution for a period of between about 1 and 60 minutes.
 43. The process of claim 24, wherein the metal substrate is immersed for a period of between about 10 to 30 minutes.
 44. The process of claim 37, wherein the ratio is selected to form said plating bath solution having a pH of about
 14. 45. The process of claim 39, wherein the magnesium substrate is immersed in the mixed solution for a period of between about 10 to 30 minutes.
 46. The process of claim 22, wherein the nickel or nickel alloy plating metal comprises a Nickel-boron plating metal, and said magnesium metal substrate comprises a magnesium substrate comprising: the first bath solution comprising: 25 to 50 g/L of said nickel chloride hexahydrate, the second bath solution comprising: 50 to 75 ml/L of said ethylenediamine, wherein said step of mixing comprises mixing the first and second bath solution components to form a mixed plating bath solution having a pH of at least
 13. 